1. Field of the Invention
This invention relates to handling indeterminate length sheet material for high speed advancement in a step-by-step manner along the length dimension. More particularly, this invention pertains to accurately positioning sheets of marginally punched continuous forms during printing in a high speed printer.
2. Description of the Prior Art
Traditionally, output printers in data processing systems have used marginally punched fan folded continuous forms fed by so-called tractors or sprocket or pin feed mechanisms. Marginally punched continuous forms may suffer such defects as (1) variations from sheet to sheet, or more importantly within a sheet, relative to corresponding hole positions; (2) variations in sheet dimensions caused by such environmental conditions as humidity; or even (3) partial destruction or separation of the marginally perforated portion from the main body of the sheets. Occasionally, manufacturing errors result in misalignment between feeding pins and holes in marginally punched forms, thereby damaging the forms.
Such defects and manufacturing errors are not generally significant enough to halt printer operation, but they do affect the spacing between printed lines, and thereby the appearance of the printed sheet. For this reason, continuous forms printing applications have been limited to those in which precision alignment of print lines within a sheet was not critical.
Pressure rollers have been used to tension and/or withdraw marginally punched continuous forms in a printer, but the forms are usually advanced past the print station by tractor pin feed mechanisms, thereby making accuracy of print alignment dependent on the location and condition of the marginal perforations.
With the development of nonimpact printing techniques, greater precision in character/symbol size and placement is possible. An array of ink jets, for example, may be accurately controlled to produce fine line or character segments for making graphs, diagrams and the like, where precision requirements are of the order of 0.002 inches (0.05 mm).
Heretofore, however, such accuracy in advancing marginally punched continuous forms has not been obtainable using traditional apparatus and methods. Thus marginally punched forms have not been used with much success in applications where the acceptability of the output is a function of fine print alignment. Such accuracy is approached in systems where paper is incrementally advanced under control of a pressure drive roll. However, with continuous forms, there is a danger of propagating alignment errors from sheet-to-sheet during a run.